The present invention relates in general to a speech communication system, and more particularly to a speech coding and decoding system which is suitable for a type of transmission, such as mobile radio communication, in which there may be many transmission code errors.
The present trend in mobile radio systems is characterized by a shift from the conventional analog system to an all digital system. In such a digital system, the speech signal is digitized and subjected to low bit rate coding. However, the bit rate is required to be less than or equal to 8kbps because of current restrictions on assigned frequencies for radio communications. Examples of a low bit rate speech coding method are described in U.S. application Ser. No. 015,025, filed Feb. 17, 1987, and assigned to the same assignee as the present application.
On the other hand, in communication systems which involve the use of a transmission medium of inherently low reliability, for example, mobile radio communication systems, error correcting codes are generally added to the transmitted data. Two different types of error correcting codes have been employed: one which is only capable of detecting a code error and another which has the additional ability to correct an erroneous code. Typical error correcting codes are discussed, for example, in Shu Lin/Daniel J. Costello, Jr., "Error Control Coding", Fundamentals and Applications, Prentice Hall, Inc., Englewood Cliffs, N.J. 07632 (1983).
However, codes having a high correcting capability tend to have a larger number of bits. In actual use, therefore, appropriate correcting codes are selected on the basis of a balance between the correcting capability and the number of bits of the code. As a result, if the extent of the transmission code error is excessively large, it cannot be corrected and remains uncorrected.
Hitherto, measures have often been adopted to employ codes having a high error correcting capability at the sacrifice of transmission efficiency and speech quality. In this regard, the correction of code errors has been the principal objective and no measures have heretofore been taken to cope with a situation where error correction cannot be effected. In particular, speech communication in which speech is transmitted by a low bit rate coding method involves the problem that, when a code error occurs, a queer or unpleasant sound is generated, thus causing a disturbance to the listener. One way of dealing with this problem is to output 0 or insert white noise in the erroneous part of the data when a code error cannot be corrected, with a view to preventing generation of undesirable sound affects. In such a case, however, a discontinuity in the speech quality is conspicuous and the speech becomes unnatural.